The Weyburn Field is located at the western periphery of the prolific Midale-Steelman oilfield trend along the northeastern flank of the Williston Basin. Hydrocarbon production is from the Mississippian Midale Beds. These bedsconstitute a small portion of a thick succession of Mississippian cyclicsediments.
The Midale Beds have been divided into the Frobisher Evaporite and the Midale Carbonate. The Midale Carbonate can be further subdivided on the basisof petrophysical well log response and general lithology into twolithostratigraphic units. These units are referred to here as the upper Marlyand lower Vuggy zones. Sediments of the Vuggy were deposited in an open marinemoderate to high energy environment, while Harly sediments were laid down in ashelf interior, restricted low energy setting. The Midale Beds dip southwardtowards the centre of the Basin. Post-Mississippian erosion has truncated theentire Midale section field. Oil is trapped by a combination of structural, diagenetic and stratigraphic elements.
Production data indicate a preferential fluid migration that is caused bythe presence of a northeast to southwest trending fracture system. Variation inthe types of diagenesis affecting both Lithostratigraphic units has led to theformation of different porosity types. The Vuggy unit is characterized by thepresence of intergranular, moldic and vuggy porosity, whereas the Marly unit isdominated by intercrystalline porosity. As a result of the varying types andareal distribution of the porosity, a significant difference in reservoircharacteristics between the two zones is apparent. Consequently, the units eachrequire a separate approach with regard to completion, production and enhancedoil recovery schemes.
